The rivalry between Uber Technologies Inc. and GrabTaxi Holdings Pte. Ltd. expanded to a new front Friday, when Grab said it is teaming up with self-driving car company nuTonomy for a wider public trial in Singapore.

Starting Friday, selected Grab customers in the city-state can use the ride-hailing app to book a ride in a nuTonomy vehicle through a special icon. All rides are free, though restricted to the one-north business district and adjacent neighborhoods.

Founded by two researchers from the Massachusetts Institute of Technology, nuTonomy began testing its self-driving taxi service in Singapore late last month, beating Uber to its own trials in Pittsburgh by days.

Like Uber's test fleet, Grab and nuTonomy will provide a backup human driver behind the wheel and a support engineer in the front passenger seat. If a trip requires travel on roads outside of one-north, the safety driver will take control of the vehicle for that portion of the trip, the company said.

"Partnering with Grab to expand our public trial in Singapore will yield valuable feedback and consumer insights as nuTonomy readies our on-demand self-driving car service for commercial launch in 2018," said Karl Iagnemma, co-founder and chief executive of Cambridge, Mass.-based nuTonomy.

Grab and Uber are aggressively competing to dominate in Southeast Asia, a market of some 600 million people.

Grab, based in Singapore, has an extensive footprint in Singapore, Malaysia, Thailand, Indonesia, Vietnam and the Philippines. It has up to 1.5 million bookings a day and hails private cars, motorcycles and helps to arrange carpooling services. On Tuesday it said it had raised $750 million in a fundraising round led by Japan's SoftBank Group Corp. Grab is now valued at over $3 billion.

Uber, meanwhile, is targeting the region as a new focus point for expansion after exiting China.

In a statement, Grab said the partnership with nuTonomy will allow the companies to better study their customers' experience with on-demand self-driving cars.

Grab also said it expects the partnership to provide new ways to improve its routing technology and mapping services, an area where it is trying to distinguish itself from Uber.

Write to Resty Woro Yuniar at restyworo.yuniar@wsj.com

(END) Dow Jones Newswires

September 22, 2016 23:05 ET (03:05 GMT)